Design decisions for Google App Engine hosted app

Question

I never used Google technologies to develop webapps before, and I'm a bit overloaded and confused. A lot of information.

My scenario: Developing a webapp for, say, company.com, using Google App Engine (Python, Webapp2, Jinja2...). The domain company.com is Google hosted (e-mail and calendars are the important parts here). So, I want my GAE-based app to add events to the domain users' calendar, send them e-mails, etc. The basic flow is:

• A user of this domain logs into the web app, using his/her mail: [e-mail]. (IMPORTANT: Users CAN'T use their private GMail accounts, or any other OpenID-based authentication. Just their domain accounts). I know this can be configurated from the admin console, when creating the app.
• The user fills a form, and a new calendar event is created. This event is added to his/her calendar, and to his/her manager's calendar too. The user receives a confirmation e-mail (event created) and the manager a notification e-mail (someone invited you...).

I've been reading a lot about authentication, authorization, google app engine and google apps, but I still don't have a clear image about what technologies/libraries/apis I should use.

I've seen a lot of examples using oAuth and Calendar API v2, but oAuth is deprecated, and seems that I should use oAuth2. Calendar API v2 is deprecated too, and v3 is recommended... I see a lot of entry points in the documentation, and I don't know where to start.

Any piece of advice is strongly appreciated. Many thanks in advance.